1. Field of the Invention
The present invention relates to a disk for a revolution sensor which is used in a steering sensor of an automobile or the like.
2. Description of the Related Art
In a steering sensor of an automobile, the steering angle or the like of a steering wheel can be detected by detecting a disk 150 for a revolution sensor which is secured to a steering shaft (not shown), as shown in FIGS. 7 and 8.
The disk 150 for a revolution sensor is formed such that an annular holding member 154 made of a synthetic resin and having a holding hole 154A is molded in such a manner as to cover the three sides of an inner peripheral portion of a disk-like slit plate 152, i.e., such that the slit plate 152 is inserted in the holding member 154. A plurality of slits 153, i.e., detection holes, are formed circumferentially at equal intervals in the vicinity of an outer periphery of the slit plate 152. Furthermore, a circular hole 156 is formed in a central portion of the slit plate 152. As shown in FIGS. 7 and 8, a pair of circular holes 155 are formed in a peripheral edge portion of this circular hole 156 in such a manner that a central angle thereof becomes 180.degree..
When the disk 150 for a revolution sensor is formed, the slit plate 152 is set in an unillustrated mold, a resin in a molten state is poured into a cavity in the mold, and the resin is allowed to solidify both in the circular holes 155 and over the three sides of the inner peripheral portion of the slit plate 152. The resin is solidified in the central portion of the slit plate 152 integrally with the slit plate 152, and is used as the holding member 154. The disk 150 for a revolution sensor is thus formed. The holding member 154 has relative-movement preventing portions 158 which are formed as the resin poured into the circular holes 155 are hardened. The relative movement preventing portions 158 are tightly secured in the circular holes 155 so as to prevent the relative movement of the holding member 154 and the slit plate 152.
However, with the conventional disk 150 for a revolution sensor, since only one pair of circular holes are formed in the slit plate 152, variations occur in the amount of thermal deformation of the resin because the thickness of the holding member 154 around the peripheral edge portion of the circular hole 156 is not uniform over the entire circumference of the circular hole 156. Accordingly, it is difficult to maintain the coaxiality of the holding hole 154A with respect to the circular hole 156 with high accuracy. In addition, it is difficult to sufficiently secure circularity because the holding hole 154A becomes elliptical, for instance. Accordingly, even if the disk 150 for a revolution sensor is mounted on a steering shaft by holding the steering shaft in the holding hole 154A whose coaxiality and circularity are not sufficient, it is difficult to detect the steering angle of the steering wheel with precision.